1. Future Challenges in Long-Distance Quantum Communication Jian-Wei Pan Hefei National Laboratory for Physical Sciences at Microscale, USTC and Physikalisches Institut der Universität Heidelberg December 15, 2005

2. or + + Classical Physics: “bit” Quantum Physics: “qubit” Entanglement: Quantum foundations: Bell’s inequality, quantum nonlocality… Quantum information processing: quantum communication, quantum computation, high precision measurement etc … Quantum Superposition

3. When information is encoded in quantum states one may outperform classical communication systems in terms of absolute security efficiency channel capacity Because quantum information systems allow encoding information by means of coherent superposition of quantum states. Why Quantum Communication?

4. Qubits: Polarization of Single Photons One bit of information per photon (encoded in polarization) Qubit: Non-cloning theorem: An unknown quantum state can not be copied precisely!

5. Bell states – maximally entangled states: Polarization Entangled Photon Pair 1-2 Singlet: where 45-degree polarization

6. Quantum Cryptographic Key Distribution Single-particle-based secret key distribution: Entanglement-based secret key distribution: [A. Ekert, Phys. Rev. Lett. 67, 661 (1991). ] [C. H. Bennett & G. Brassard, BB84 protocol (1984) ]

7. Quantum Teleportation Initial state The shared entangled pair where [C.H. Bennett et al., Phys. Rev. Lett. 73, 3801 (1993)]

8. Entanglement Swapping [M. Zukowski et al., Phys. Rev. Lett. 71, 4287 (1993)]

9. achieved distance: 100km fiber-based (Toshiba Research Europe) 23km free-space (TU Munich) Key Distribution with Single Photons [C. Kurtsiefer et al., Nature 419, 450 (2002)]

10. Generation of Photonic Entanglement [P. G. Kwiat et al., Phys. Rev. Lett. 75, 4337 (1995).]

11. Key Distribution with Entangled Photons achieved distance: 1km for both fibre-based and free-space Fibre: [T. Jennewein et al., Phys. Rev. Lett. 84, 4729 (2000).] [D. S. Naik, et al., Phys. Rev. Lett. 84, 4733 (2000).] [W. Tittel et al., Phys. Rev. Lett. 84, 4737 (2000).] Free-space : [ M. Aspelmeyer et al., Science 301, 621 (2003). ]

12. Experimental Quantum Teleportation Teleportation: [D. Bouwmeester & J.-W. Pan et al., Nature 390, 575 (1997)] The setup Entanglement Swapping: [J.-W. Pan et al., Phys. Rev. Lett. 80, 3891 (1998)] The result

13. Our dream: achieving long-distance quantum communication!

14. However, due to the noisy quantum channel photon loss (1) absorption (2) decoherence degrading entanglement quality Difficulties in Long-Distance Quantum Communication Free-Space Distribution of Entangled Photons

15. Free-Space Distribution of Entangled Photons over 13km [C.-Z. Peng et al., Phys. Rev. Lett. 94, 150501 (2005)] Free-space entanglement distribution - we are working on 20km and 500km scale…

16. Entanglement swapping: solution to photon loss: [N. Gisin et al., Rev. Mod. Phys. 74, 145 (2002)] Entanglement purification: solution to decoherence [C. H. Bennett et al., Phys. Rev. Lett. 76, 722 (1996)] [D. Deutsch et al., Phys. Rev. Lett. 77, 2818 (1996)] Another Solution to Photon Loss and Decoherence

17. Generating Entangled States over Long-Distance Quantum repeaters: [H. Briegel et al., Phys. Rev. Lett. 81, 5932(1998)] Require entanglement swapping with high precision entanglement purification with high precision quantum memory

18. Experimental Entanglement Purification and Swapping Before purification, F=3/4 After purification, F=13/14 [J.-W. Pan et al., Nature 423, 417 (2003)] [J.-W. Pan et al., Nature 410, 1067 (2001)] [J.-W. Pan et al., Nature 421, 721 (2003)]

19. Drawback in Former Experiments Probabilistic entangled photon source Probabilistic entanglement purification Bad weather Quantum memory In N -stage realization, the cost of resource is proportional to With the help of quantum memory, the total cost is then

20. Storage of single-photon states in atomic ensembles [C. Liu et al., Nature 409, 490 (2001)] [D. F. Phillips et al., Phys. Rev. Lett. 86, 783 (2001)] Storage of light in atomic ensembles motivate [L.-M. Duan et al., Nature 414, 413 (2001)] Solution with Atomic Ensembles

21. Entanglement Generation Maximally entangled in the number basis!

22. Entanglement Connection Steps ： 1.Apply a reverse read laser pulse to transfer atomic excitation to optical exc. 2.Succeeds if D1 or D2 registers one photon 3.Fails otherwise, and repeat every step from entanglement generation

23. The most recent experiment results Observation of Stokes and anti-Stokes photon Harvard: M. D. Lukin… [C. H. Van der Wal et al., Science 301, 196 (2003)] Caltech: H. J. Kimble… [A. Kuzmich et al., Nature 423, 731 (2003)] Gatech: A. Kuzmich… [ D. N. Matsukevich et al., Science 306, 663 (2004 )] Heidelberg: J.-W. Pan … long-life time quantum memory [S. Chen et al., in preparation for Phys. Rev. Lett.] working on a phase insensitive scheme… Synchronization of two independent lasers USTC: J.-W. Pan, J. Zhang and Z.-Y. Wei … [T. Yang et al., submitted to Phys. Rev. Lett. (2005) ]

24. |Photons> |Atoms> + Powerful Quantum Superposition Promising Long-Distance Quantum Communication